But gain aside, I still don't quite understand the need for an outboard crossover to control the frequency between the HF and LF sections since both amps are feeding full range and each section of the speaker still operates within its own design frequency range.kalali,
the advantage of an external x-over lies in the fact that you can tweak the x-over slopes to suit your listening needs &/or your room.
In an in-built (in the speaker cabinet) x-over the designer does this job for you using his design skill & keeping in mind 99% of the population so that his speakers work in a wide range of room sizes. So, it's often the best compromise.
With an external x-over you can change the slope (-6dB or -12dB or -24dB or -48dB & so on) make the speaker tuned to your room.
Plus, some of the factory x-overs are very power hungry. Using an ext x-over you can overcome this shortfall & make it easier for your amp to drive your speakers.
You can also upgrade all the components in your ext x-over & make it as transparent sounding as your wallet will allow.
Plus another major thing these days - some digital x-overs will also allow you to do room correction & bring your speaker closer to being a time-coherent speaker (which is a big deal, i.e. being time coherent). The merits of room & speaker correction towards making the speaker time-coherent are huge. One listen & you will know.
hope this clarifies.....